Public Transportation

Public transit, school and intercity transit buses are substantially powered by diesel.

Public transportation is a valuable asset to every community around the country and the nation as a whole, enabling personal mobility and supporting economic prosperity. The principal mission of public transportation agencies is to provide efficient mobility services for its constituents, foregoing the need for personal vehicle trips that contribute substantially to traffic congestion and urban air pollution.

In addition to regular route services, assets of public transportation agencies are often part of disaster response plans, such as transit buses enabling evacuation of impacted areas and serving as temporary shelters.

Diesel is the predominant technology powering public transit, school, and intercity bus services nationwide because of its safety, reliability, efficiency, durability, and now near-zero emissions.

According to the American Public Transportation Association (APTA), in 2020, diesel and biodiesel were the predominant fuel sources for transit buses, followed by natural gas and hybrid diesel electric.

The latest data shows that as of December 2021, 79% of all transit buses in operation were powered by a diesel engine; up 3% from 2020. Of those, 47% are of the latest generation of advanced diesel technology that achieves near zero emissions; 98% fewer particulate emissions and 98% fewer emissions of nitrogen oxides, compared to previous generation buses.

Advanced technology diesel was first introduced in transit buses in 2010 and has been the standard for all new buses since that time. Achieving near zero emissions is enabled by new particulate filters and selective catalytic reduction technology that reduces nitrogen oxides to other compounds.

Diesel-hybrid electric technology was introduced into transit buses in the mid-2000’s and has been refined continually since that time. These dual system buses enable the use of a downsized diesel engine combined with the benefits of parallel, or serial, hybrid systems that capture and store energy from braking and utilize that to reduce the load on the diesel engine. This reduces fuel consumption, greenhouse gas emissions, and brake wear ­ – a major cost for transit agencies.

The use of advanced renewable biobased diesel fuels is an increasingly successful sustainability and decarbonization strategy for transit fleets. All diesel engines  –  in new and existing transit buses can – using biodiesel blends of up to B20.

Advanced diesel technology buses today compete with battery electric, hydrogen fuel cell and natural gas technologies. The primary basis for introducing the new fuels is decarbonization of the transportation sector to achieve greenhouse gas reduction and broader climate goals. Federal government funding and incentives are increasingly available to offset the significantly higher costs of the vehicles and the attendant infrastructure necessary to fuel or charge the vehicles.

When comparing fuel and technology choices, transit fleets must consider a myriad of options that best serve their individual needs. There are various tradeoffs with all fuels and technologies, and transit agencies are best equipped to determine which fuels and technologies can best serve their communities and the mission of their transit agencies.

In addition to the driving range advantages of diesel over other fuels, diesel provides maximum flexibility for changing routes, service runs and expansions into new communities without limitation of cost of installation or availability of fueling network or access to parts and servicing. For example, electric transit buses operating in climate extreme areas (hot or cold) often experience reduction in driving range due to energy being diverted to power heaters or air conditioning systems. In some instances, this could require rerouting of buses or the use of reliever buses to fill in the gaps to sustain service uptime commitments.  

Because most all transit buses are purchased with assistance from the Federal Transit Administration, they are expected to be in service for 22 years. With limited funding and desire not to raise fares or curtail services, often a community will get more clean air for the dollar and more newer buses in the fleet by investing in the newest generation of advanced diesel technology rather than more expensive alternative fuel options.

Advanced diesel technology buses are 20 to 25% less expensive than compressed natural gas (CNG) buses, and two or three times less expensive than all-electric models. Advanced diesel technology buses do not require new and separate fueling, or recharging, infrastructure required for electric or hydrogen options.

Replacing older buses with new advanced diesel technology buses can help transit agencies achieve greater emission reductions for the communities they serve, while providing reliable and low-cost transit needs. And when coupled with using renewable low carbon biodiesel fuels, transit fleets can rapidly take steps to decarbonizing their operations while maintaining fare levels and service at the lowest cost.